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Stringlike Cooperative Motion Explains the Influence of Pressure on Relaxation in a Model Glass-Forming Polymer Melt
Wen-Sheng Xu, Jack F Douglas1, Karl F Freed
1Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States.
ACS Macro Letters
|June 2, 2022
Summary
High pressure significantly alters polymer melt dynamics. This study reveals pressure-dependent structural relaxation times and fragility in polymer melts, explained by the string model.
Area of Science:
- Polymer Physics
- Materials Science
- Computational Chemistry
Background:
- Experimental studies show pressure profoundly impacts glass-forming polymer melt dynamics.
- A complete microscopic understanding of these pressure effects is currently lacking.
Purpose of the Study:
- To investigate the structural relaxation of a model glass-forming polymer melt under varying pressures.
- To elucidate the microscopic mechanisms governing pressure-induced changes in polymer dynamics.
Main Methods:
- Molecular dynamics simulations were employed to model a glass-forming polymer melt.
- Simulations covered a wide range of applied pressures (P).
Main Results:
- The pressure dependence of structural relaxation time (τ_α) followed a pressure analog of the Vogel-Fulcher-Tammann equation.
- Characteristic glass formation temperatures increased with pressure, while fragility decreased.
- The string model of glass formation quantitatively described τ_α across different pressures.
Conclusions:
- Pressure significantly influences the structural relaxation dynamics of polymer melts.
- The findings align with experimental observations and the generalized entropy theory.
- The proportionality of activation enthalpy and entropy, as predicted by the string model, offers insights into polymeric materials under external fields.
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